An apparatus for radio communications having an imaging device with a flash, such as a camera mobile phone, typically has a flashunit based on one or more high power LEDs. High power LEDs are driven by high power drivers. The combination of a high power LED and a high power driver can offer a very competitive flash solution for mobile imaging devices. Known achieved 4-6 W power peaks in these flash solutions can push the camera operational range up to 2-2.5 meters in the dark. However, having radio connection capability and a camera flash in the same device typically causes problems. A simultaneous radio transmission and use of the flash may, especially when such efficient flashes are used, overload the safety circuit in the battery. Even with lower power flashes, image quality suffers when a camera flash and a radio transmission require power simultaneously. In order to compensate the power needs, an extra condensor or capacitor for storing charge for delivering current for power peaks can be employed. Alternatively, a more effective battery can be used for better satisfying the increased power needs. But such solutions generally affect the overall design, increase size and weight of the device, and are thus not desired.
One solution in order to avoid problems is to keep a LED flash current at such a low level that operation with a simultaneous transmission is possible. However, in this way the flash performance is typically left insufficiently low.
In order to be able to enhance the flash performance and reduce high current peaks from the battery, the transmission operations should be synchronized with the flash. In one solution accomplishing the synchronization the driver circuit has a dedicated input pin for masking. Masking is done during transmission and it disables the flash LED driver during the current demanding phase of the power amplifier controlling the transmission. However, masking the LED current creates a further problem. When the LED current is altered, i.e. chopped during an image exposure, the image will have dark, generally horizontal stripes. An example of such an image having horizontal stripes 100 is presented in a FIG. 1. If transmission occurs during the exposure of certain rows of the image, the current is directed for the transmission, thus the flash is disabled. The unpowered time of flash makes the captured image unequally exposured. The resulting image then has dark stripes 100 in corresponding rows, as shown in FIG. 1.
The disclosed image quality problems are present when a rolling shutter is used. A rolling shutter typically exposes the image row by row. The exposure thus proceeds typically starting from the first, uppermost row. After the first row is exposed, the exposure is accomplished for the next row. And similarly each row of the image is exposed, one by one, in the corresponding row order.